Feature Articles

Chiral Chemistry Aids Drug Development

Fewer Side Effects Are One Benefit of Enhancements of this Technology

Chiral technology has advanced from the days of Pasteur's separations of single enantiomers by tweezers to automated systems and chiral catalysts, with the result that most drugs today are synthesized from single isomers. Consequently, dangerous or inactive mirror images of certain molecules can be identified so early in the development process that they often needn't be synthesized.

The importance of this capability is exemplified by thalidomide, used in the 1950s to decrease morning sickness. One enantiomer did just that. The other, however, caused birth defects. Nowadays, regulatory bodies require testing data on both enantiomers, greatly improving safety.

This is important for developing drugs, but may also have value for compounds coming off patent. "Most of the drugs coming off patent in the next five years were developed before chiral catalysis was practical," comments Mike Cannarsa, Ph.D., vp of business development at VioQuest Pharmaceuticals (www vioguest .com). "There is an opportunity for new markets to develop more effective processes, and thereby lower manufacturing costs," Dr. Cannarsa says.

VioQuest Pharmaceuticals, through its subsidiary, Chiral Quest (www.chiralquest.com), developed a practical, scaleable chiral chemistry to streamline separations. That chemistry has resulted in some catalytic processes that have never been done before, he says, citing the Lipitor intermediate as one example.

"Up until now, it was a biocatalytic process," using enzymes rather than chemical catalysts. The benefit, explains Dr. Cannarsa, is that the product is more concentrated and "is more efficient by a factor of five."

So far, Dr. Cannarsa says, "we've identified five of the catalysts that are broadly applicable to A-to-B conversions," and, he adds, they can be made cost-effectively. Currently, one pound of VioQuest's catalyst makes 1,000 pounds of product.

"Because of the complexity of the target drug molecule, you need a family of catalysts that chemists can screen against their molecule," he says. He estimates that those five can meet about 70% of a chemist's hydrogenation needs.

As Dr. Cannarsa recounts, of 15 compounds in a particular pipeline, these chiral catalysts "worked for 10 or 11 of them." Chiral Quest is working to expand the family by one to two chiral catalysts per year.

Achiral and Chiral Molecules

"When molecules with asymmetric centers are synthesized or purified through enantioselective chromatography, it is critical to understand which compounds correspond to which enantiomers, and to know which ones are biologically active, or eutomers and which are not, called distomers," notes Oliver McConnell, Ph.D., director in the chemical and screening sciences division.

In a recent presentation at "Chirality 2005" in Parma, Italy, Dr. McConnell reported that phenylglycidols with different substituents in the phenyl group in the synthesized molecules did not grossly alter spectroscopic features.

Therefore, "diagnostic absorption bands in the respective vibrational circular dichoisum spectra, the sign and shape of the observed electronic circular dichoisum curves, and the sign and magnitude of the calculated versus observed specific optical rotations could be used to determine the absolute stereochemistry of analogs without necessarily requiring time-consuming calculations of all low-energy conformers of all compounds."

"What's important," Dr. McConnell elaborates, "is that in this study, you're able to track absolute stereochemistry to the final product."

Calculated versus observed vibrational circular dichoisum (VCD) and optical rotation are newer procedures for determining absolute stereochemistry that are gaining adherents, he says. This is because they lack the drawbacks of the more traditional methods, such as NMR, "which typically requires derivatization in the so-called Mosher's approach," and x-ray crystallography, which requires the molecule to be crystallized, and "may be difficult to make," says Dr. McConnell.

In comparison,VCD and optical rotation may be relatively fast and do not require derivatization. "Measurements for enantiomers of one small molecule take about a day to a day and a half," versus several days of calculation on a high-powered PC. Low molecular weight and rigid molecules with few conformers, key chiral intermediates common to an elaborated parallel synthesis library, are ideal candidates.

Chiral Chromatography

PDR-Chiral (www.pdr-chiralcom) is using chiral chromatography to provide chiral separations as early as possible in compound development. The goal, according to Gary Yanik, president and CEO, is to work smarter, using as little labor as possible to achieve separations. That requires automation.

"Ten years ago," Yanik says, "chromatography was a bad word as far as synthesis chemists were concerned. They wanted to brew molecules without chromatography.

"Now they're realizing that some of the steps in processing can be accomplished by chromatography much more economically than by basic chemistry. Earlier involvement equals better efficiency," Yanik emphasizes. Therefore, "Chromatography is being used more creatively, and earlier in products."

"The way you have been doing it isn't necessarily the way you should be doing it," Yanik stresses. In his presentation at "Chirality 2005", he emphasized the need to configure systems for full automation, tying together the appropriate solvent mixers, column selectors, injectors, detectors, collectors, and software.

Beyond that, however, researchers need to take the time to optimize their methods, techniques, and procedures, writing them down, so they are computercontrolled and self-maintaining. "People don't do this often enough," he says.

"People need to develop a flow-chart mentality for processing compounds in the minimum amount of time, but they don't see it as a flow of work and so haven't developed a system or procedure" to maximize workflow.

Yanik says for example,"they may do a literature search. But in 24 hours we can screen against compounds and probably learn more than they've found in the literature, because these compounds have just been created. They, in comparison, are researching similarities. That's not necessarily productive."

In terms of increasing productivity, he recommends screening using gradients for increased speeds and varying the columns, but keeping the eluent the same, with sequences to proceed in increasing or decreasing miscibility values, including wash and equilibrate methods at the end. "That's important," he says, "because if you don't pay attention, the solvents won't dissolve in each other."

In prep purification, Yanik recommends making a large number of injections to achieve low-risk, high-purity separations. The goal, is to select the highest loading possible while maintaining baseline separation and the shortest cycle time possible that can avoid impurities, .

Supercritical fluid chromatography (SFC) is a new technology that is gaining popularity and can help reduce evaporation volume. "Normally, researchers use hexane in solution, but by using SFC, we can replace the hexane with carbon dioxide and alcohol, which is safer for the environment. When you finish a job, it's already evaporated, and it is easier to recover the compound," Yanik says.

PDR-Chiral's latest software products include the Auto MDS for method development and AutoPrep for purification. These modules can be integrated into other systems, which then can be operated through PDR-Chiral software for 24/7 operation.

Identifying Peptide Differences

Astec (www.astecusa.com) discovered it could identify differences in peptides as small as a single amino acid or the chirality of a single amino acid, notes Thomas Beesley, CEO. That technology is helping scientists use amino acids to, for example, "avoid a particular protein binding site," by changing the amino acid's enantiomic form from L to D.

Compared to UV detection and other popular methods, this offers a 1050% improvement in reliability, he says. It offers the ability to reliably identify enantiomeric pairs and elution orders. The Chiralyzer goes at the end of the column and can work in conjunction with mass spectrometry. That combination allows researchers to identify the molecule and differentiate chirality.

Astec recently released the Chirobiotic V2 and T2 chiral chromatography columns that offer 2- to 20-fold increased capacities over the standard Chirobiotic phases. The performance has been noted, especially in both the polar organic and polar ionic modes, Beesley comments, and has been designed for use in the optimization process after selectivity screening.

"We've been developing a novel polymeric technology to enhance the capacity of the stationary phase to obtain pure enantiomers from a mixture with good economics," Beesley notes.

In this P-CAPDP phase, a bifunctional group is catalyzed to create long repeat chiral moieties. The P-CAPDP underwent validation tests in early autumn and has just been released. "It is viewed as an ideal chiral phase for SFC applications," Beesley explains.

Pure Enantiomers

"From the point of view of pharmaceutical R&D, it is critically important to market pure enantiomers," notes Scott Biller, Ph.D., head, global discovery chemistry, Novartis Institutes for Biomedical Research (www.nibr.novartis. com).

Dr. Biller says that an important focus in modern organic chemistry is the design of chiral catalysts to take "achiral moieties and connect them together to provide a single enantiomer. An exciting area is asymmetric organocatalysis.

"Early examples of asymetric organocatalysis were published in the 1960's, but this has only recently become a hot area," Dr. Biller notes. At the research scale, Novartis is using such work to make single chiral isomers.

Likewise, Celgene (www.celgene.com) gained approval last May for Focalin XR (dexmethylphenidate hydrochloride), an extended-release, single-enantiomeric version of Ritalin. By containing only the active d-isomer, attention deficit/hyperactivity disorder patients can take half the dose recommended for Ritalin and expect fewer side effects. Global marketing rights, excluding Canada, were licensed to Novartis.

During the past 10 years, chiral structures have become increasingly important, particularly as enantiomers were substituted for racemic versions, Dr. McConnell says.

As further evidence, the U.S. market for chiral compounds is growing at an annual growth rate of 8.8% and is expected to exceed $1.8 billion by 2008, up from $1.2 billion in 2003, according to the most recent market study, a report released by BCC last year.

Of that, chiral manufacturing dominates the market, accounting for more than $1 million in 2003 and is expected to top $1.6 billion by 2008.

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